High-speed serial communication is performed as a method for transferring large volume data between Large Scale Integrated circuits (LSIs) or between network apparatuses. In recent years, a transmission standard of the high-speed serial communication having a transmission speed of 56 Gbps has been developed in Optical Internetworking Forum (OIF) or the like. In the transmission standard, employment of a multilevel transmission technique referred to as Pulse Amplitude Modulation 4 (PAM4) instead of Non Return to Zero (NRZ) has been proposed.
NRZ is a technique of dividing signal potentials into a high (H) level and a low (L) level and transmitting the signal potentials of the H level and the signal potentials of the L level which are associated with one-bit logical value “1” and “0”, respectively. On the other hand, PAM4 is a technique of dividing signal potentials into four potential levels and transmitting the signal potentials in the four potential levels which are associated with respective two-bit logical values. In PAM4, when an amount of information in NRZ is to be similarly transmitted, a baud rate of a signal may be reduced to half. The higher a frequency of a signal to be transmitted becomes, the larger a loss of a transmission path, such as a printed circuit or a cable, becomes. If the baud rate is lowered, a signal band (a Nyquist frequency) is lowered, and therefore, a transmission loss may be reduced.
Note that, in PAM4, use of a gray code in association between the two-bit logical values and the four potential levels has been proposed. The gray code is characterized in that a signal distance between adjacent codes is 1. In a case where four potential levels are denoted by L0, L1, L2, and L3 in ascending order, “00”, “01”, “11”, and “10” are associated with L0, L1, L2, and L3, respectively. Since the gray code is used, a case where a lead error to an adjacent code which occurs on a reception side since a signal potential is changed due to application of noise at a time of signal transmission is recognized as two-bit error is suppressed.
Meanwhile, in general PAM reception circuits which receive a signal of PAM4, three comparison circuits which compare the received signal with individual three threshold values are used to determine one of the four potential levels which correspond to the received signal. Therefore, in the PAM reception circuits, the number of comparison circuits is larger than the number of reception circuits which receive signals of NRZ, and accordingly, power consumption is also increased.
The followings are reference documents.
[Document 1] Japanese Laid-open Patent Publication No. 10-302486 and
[Document 2] Japanese National Publication of International Patent Application No. 2001-516932.